ftplibpp | Platform independent c++ library providing ftp client | FTP library

I'm currently adding ftp functionality to a project via this library. Its one optional dependency is OpenSSL, which adds the more specific encryption functionality I'm looking for. I've pulled down both, and built and installed and added as additional includes/dependencies the later, which all lines up with this answer here. However, whenever I go to build my project, any call to OpenSSL functions or macros are deemed 'identifier not found' (the only place these are called to is ftplib.cpp).

In attempts to resolve this:

• I've changed the include path to an absolute one (which shouldn't make a difference as the relevant environment variable is already included in the project's 'Additional Include Directories').
• I've rebuilt the whole solution (which only consists of the one project anyway) to regenerate any pre-compiled headers.
• I've double-checked that all the identifiers can be found in the ssl.h header included.
• Checked that OpenSSL doesn't use any namespaces.
• Preprocessed the offending file, which does contain the declarations of these identifiers (as seen in the ssl.h header)!
• I've successfully compiled/built the preprocessed source file, although wouldn't debug complaining about a missing dll.

This final test tells me that, in theory, everything should be working? If anyone else has any ideas or insights I'd be very grateful!